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Plant power

FUEL made from plants could soon be providing cheap energy for rural areas in
developing countries. Researchers in the US have found a way to generate much
more concentrated gas from farm waste.

Heating biomass breaks it down into gases such as methane and carbon
monoxide. Traditional gasifiers generate the necessary heat by burning the
biomass in air. Unfortunately the nitrogen in air dilutes the gas produced. “The
gasifiers typically produce a gas with a heating value that’s only 4500
kilojoules per cubic metre,” says Robert Brown of the Center for Coal and the
Environment at Iowa State University in Ames. “That’s roughly an eighth of the
heating value of natural gas.”

By separating the burning and heating processes, Brown and his team have
produced gas with a heating value of 14 800 kilojoules per cubic metre. In the
prototype gasifier, which is a tube 0.5 metres in diameter and 2.5 metres tall,
the biomass is first burnt in air for about eight minutes. The heat from this
process is absorbed by metal ballast, which reaches temperatures of up to 760
°C.

Next, steam generated externally is forced into the cylinder to transfer the
heat from ballast to biomass. Cooling the mixture makes the steam condense,
leaving a gas rich in methane, hydrogen and carbon monoxide, but largely free of
nitrogen. Brown expects the final version to be just as good at producing gas as
other methods such as anaerobic digestion and fermentation.

Gary Staats, a fuel development specialist with the US Department of Energy,
says the simplicity of the gasifier boosts its appeal. “It’d be a real positive
[development] in reducing the cutting of trees to produce fuel,” he says. The
gasifier could be particularly useful in India, which produces enormous amounts
of bagasse, the fibrous residue left after sugar cane has been processed.